1. Field of the Invention
The present invention relates generally to inertial type transducers capable of converting energy between electrical and mechanical form and, more particularly, to inertial type transducers that utilize a plurality of co-axially aligned moving coils and methods of using such transducers.
2. Description of the Related Art
Inertial voice coil actuators may be used to acoustically stimulate semi-rigid structures to reproduce sound. Various types of electro-mechanical transducers may be attached to structures that are characterized by a relatively high mechanical input impedance, such as room partitions, ceilings, furniture, etc., and that then act as a soundboard when acoustically-stimulated to radiate sound. Efficient coupling between the electrical stimulus and sound output may be made with electro-mechanical transduction machinery that is designed to create high force for a given electrical input.
The electro-acoustic transducers (or systems) used for acoustic sound reproduction may include: solid state, solenoid, moving magnet and moving voice coil transducers.
Solid state transducers may use piezoceramic or magnetostrictive materials as their core. These materials exhibit physical shape change properties when exposed to an applied electric or magnetic field. These devices in acoustic applications are characterized by high mechanical output impedance but with very limited displacement. Their use is most common in high frequencies above 200 Hertz (Hz). Commercial use is typically limited by distortion related to the intrinsic material properties.
Solenoid transducers are generally not suitable for high fidelity sound reproduction applications. Some of the earliest attempts to commercialize inertial type acoustic transducers utilized solenoid type armatures within a fixed electromagnet. These systems are characterized by low frequency operation. High frequency operation is often limited by magnetic core saturation or eddy current distortion.
Moving magnet transducers, although capable of very high efficiency in narrow frequency ranges, have shown little commercial viability for full-frequency, high fidelity applications. They share similar physical constrains as those of solenoid transducers.
Most of the commercial attempts for sound reproduction have been based on the moving voice coil transducer architecture that may be used for conventional loudspeaker applications. These systems are characterized by relatively low force and high displacements.
As is well known in the art, the force generated by an electro-dynamic transducer is a product of the current, i, length of coil wire, L, and magnetic flux density, B, so that F=iLB. The length of the coil wire that is within the annular magnetic gap is defined as the length, L. This force is what creates the movement of the coil and subsequently generates sound.
These inertial type voice coil transducers are built upon magnetic circuit designs that have classically been used for conventional cone type loudspeakers and not optimized for driving soundboard type structures. These voice coil actuators often require the use of an external housing to support the heavy magnet assembly relative to the voice coil. The voice coil is in communication with the external housing at a location coincident with an acoustic output system that permits the transducer housing to be mechanically attached to a soundboard.
Prior loudspeaker motors include a magnet circuit assembly having a permanent annular magnet, polarized in the axial direction, and sandwiched between two magnetizable plates. One of the plates carries a cylindrical post that extends through a central space defined by the annular magnet, generally referred to as a cylindrical pole piece. The other plate has an annular opening, somewhat larger than the diameter of the pole piece, such that an annular magnetic gap is formed between the post and the inner edge of the associated annular plate. The height of the gap is formed by the thickness of the annular plate having the annular opening.
The basic architecture of the loudspeaker motor design is based upon low magnetic energy magnets, typically comprised of ceramic materials. In order for sufficient magnetic flux to be generated within the annular magnetic gap, the annular magnet must be very large relative to the other components. Some manufacturers have utilized higher energy rare earth based magnets such as Neodymium, but this magnetic architecture is not optimized for the characteristics of these magnets.
Voice coil actuators have a moveable voice coil disposed within the annular magnetic gap. For speakers that use a large body such as a wall to generate sound, the coil has a suspension system that typically utilizes an external housing to which the annular magnet and magnetizable plates are also attached. The external housing provides radial stiffness and axial compliance to the coil. The moving coil has a first end fixedly secured to a radially central portion of the inner surface of the external housing wall. A mounting screw secured to an exterior well portion of the exterior housing may be attached to the wall.
Patents that disclose some of the aforementioned factors include U.S. Pat. No. 2,341,275; U.S. Pat. No. 3,609,253; U.S. Pat. No. 3,728,497; U.S. Pat. No. 4,297,537; U.S. Pat. No. 4,951,270; U.S. Pat. No. 5,335,284; and U.S. Pat. No. 5,473,700.
In practice, the annular magnet, magnetizable plates, external housing and structural attachment point comprise a system that is large and heavy relative to the total dynamic force the actuator is capable of generating. If the external housing is mounted on a vertical facing surface, e.g., a wall, large bending moments are placed on the structural attachment point and the housing must accommodate these moments without translating them to the coil.
U.S. Pat. No. 6,618,487 describes an electro-dynamic inertial exciter that is characterized by a magnetic circuit, which is mechanically clipped to a carrier assembly, which integrates an annular voice coil carrier and an axially compliant suspension. The voice coil carrier and suspension may be formed from co-molded plastics.
U.S. Pat. No. 7,386,137 describes an electro-dynamic inertial exciter that is characterized by a symmetric dual motor concept, wherein two magnetic circuits are symmetric about a mirror plane. Interposed between the two magnetic circuits is a common voice coil former coupled to an elongated shaft. The elongated shaft rides on friction bearings, while providing radial alignment of the voice coils within their respective air gaps.
U.S. Pat. No. 7,386,144 describes a momentum type transducer that utilizes a single voice coil operating in an air gap with radially polarized magnets. The magnetic circuit is aligned with the moving voice coil via a plurality of suspension elements between the magnetic circuit and the moving voice coil.